1. Field of the Invention
This invention relates to an electrophotographic record apparatus for rendering an image visible using coloring particles of toner, etc., such as a printer, a facsimile, or a copier and to a control method thereof.
2. Background Art
A control method an electrophotographic record apparatus in a related art will be discussed. A record apparatus using electrophotography includes a print photo process of rendering coloring particles visible on the surface of a record body as an image and a fixing process of transferring the coloring particle image rendered visible to the record body and fixing the image. Powder called toner dedicated to electrophotography is used as the coloring particles. In the charging step, the full surface of a photoconductor is once charged and subsequently in the light exposure step, light is applied, thereby partially discharging. A potential contrast based on the charge area and the discharge area is formed on the surface of the photoconductor and is called an electrostatic latent image.
In the developing step following the light exposure step, first the toner images of coloring particles are charged. As the toner charging method, a dual-component developing method using carrier beads or a mono component developing method of charging by friction with a toner member, etc., is available. On the other hand, to render an electrostatic latent image visible, a method called bias developing is often used. In the bias developing, a bias voltage is applied to a developing roller and toner particles charged by the action of an electric field occurring between the latent image potential formed on the photoconductor surface and the developing roller are separated from the developer on the surface of the developing roller and moved to the photoconductor surface for forming an image. The above-mentioned charge potential or discharge potential may be used as the latent image potential (namely, the potential of the image formation portion of the photoconductor). Generally, the method of using the charge potential as the latent image potential is called normal developing method and the method of using the discharge potential is called inverse developing method.
The charge potential or discharge potential, whichever is unused as the latent image potential, is called background potential. The bias voltage of the developing roller is set midway between the charge potential and the discharge potential, and the difference between the bias voltage of the developing roller and the latent image potential is called developing potential difference. Likewise, the difference from the background potential is called background potential difference. If the developing potential difference is large, the formed electric field (called developing electric field) becomes strong and thus the developing performance is enhanced. On the other hand, the background potential difference affects the quality of the background portion of an image and if the background potential difference is small, fogging to the background portion increases.
By the way, the charge amount of toner has close relationship with the developing performance. As the charge amount of toner is large, the developing performance in the same developing electric field strength is degraded.
Next, varying of the developing performance with time will be discussed. The varying factors of the developing performance with time are roughly classified into those of the charge amount of a developer with time and those of the developing electric field with time. Generally, the charge amount of toner easily varies with humidity in the atmosphere; as the humidity is low, the charge amount increases and as the humidity is high, the charge amount decreases. With the dual-component developing, the surface state of toner and carrier beads changes with time due to friction with an agitation member, etc., and the charge amount of toner changes.
On the other hand, it is known that the potential of the discharge area formed by light exposure varies with temperature change of a photoconductor, humidity change of the atmosphere, or film thickness change with time although the light exposure amount is constant. The potential of the discharge area varies markedly when an intermediate potential area incompletely discharged is provided in such a manner that insufficient light exposure amount is given.
By the way, for line images, dots, etc., the influence of the edge effect of an electric field is large and the strength of the developing electric field is larger than that of the developing electric field (parallel electric field) occurring in the inside of a solid area (solid image part). Hereinafter, the electric field having larger strength than the parallel electric field because of the influence of the edge effect will be referred to as peripheral electric field.
As the film thickness of a photoconductor changes with time, the strengths of the parallel electric field and the peripheral electric field change and the strength ratio between the parallel electric field and the peripheral electric field also changes even under the same developing potential difference.
To make the image quality constant over time, it is necessary to correct change in developing performance and variation in the developing electric field caused by variation in the charge amount of the developer described above.
To correct the density variation of a print image occurring with change in the charge amount of the developer, the following method is known: Before an image is formed, a test patch latent image is formed on a photoconductor by a laser write unit and is developed by a developing machine to form a test patch image and then the reflection density (image density) of the test patch image is measured by an optical image density sensor placed upstream from a cleaning unit and if the reflection density (image density) differs from the default value, the developing bias, the charger grid voltage, toner replenishment, and the like are controlled.
As a correction method of variation in the developing electric field, the following method is known: The potential on the surface of a photoconductor is detected by a potential sensor and the film thickness of the photoconductor is detected by some method and the laser light amount is changed so as to make the developing electric field constant for controlling the potential on the surface of the photoconductor, for example, as described in JP-A-11-15214.
Further, a method of using both control of the optical image density sensor and control of the photoconductor potential sensor as mentioned above is also known.
In the related art described above, the light exposure amounts in the line image and the dot image part wherein developing of a peripheral electric field is mainly performed and the solid area (solid image part) wherein developing of a parallel electric field is mainly performed are changed at the same time and thus if the image quality in the line image and the dot image part takes precedence over that in the solid area for stabilizing the image quality, the density of the portion developed by the parallel electric field at the center is lowered in the solid area (solid image part) wherein the parallel electric field and the peripheral electric field are mixed; this is a problem.
It is an object of the invention to provide an image formation apparatus for keeping secularly stable image quality for all image patterns if change in the charge amount of a developer caused by environment variation or over time or change in the photoconductor characteristic occurs in electrophotographic record apparatus.
To accomplish the object, a test patch image formed on a photoconductor by a parallel electric field is detected by an image density sensor, the developing bias and the photoconductor surface potential are controlled so that the density detected by the image density sensor becomes constant, a test patch image formed on the photoconductor by a peripheral electric field is detected by the image density sensor with the control condition of the image density sensor maintained, and the light application amount in which an image area of a several-dot width adjacent a blank paper part is exposed to light is controlled so that the density detected by the image density sensor becomes constant.